4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 93 



the filter. The angular position of a given " undesired " color about 

 the axis of the filter depends upon two factors: i, the difference in 

 the indices of the liquid and glass for that wave length ; and 2, the 



4500 



5000 



65O0 



7000 7500 



5500 eooo 

 WAVE LENGTH 



Fig. 2. — Transmission curves of a set of five filters at 20° C. 



number of interfaces through which the beam passes (i. e., particle 

 size and thickness of the filter). Also, since the interfaces are oriented 

 in a random or probability manner, there exists only a " most proba- 



k 



